1. Field to the Invention
The present invention relates to a golf club shaft and more particularly to a golf club shaft, made of a fiber reinforced resin, which has an improved strength without increasing its weight.
2. Description of the Related Art
In the case where a prepreg in which fibers are impregnated with a resin is used as the material of the golf club shaft, it is possible to wind on a mandrel a so-called straight layer whose reinforcing fibers are parallel with the axis of the golf club shaft and an angular layer having reinforcing fibers at a certain angle with respect to the axis of the golf club shaft.
In recent years, as the head of a golf club becomes larger and its shaft becomes longer, the shaft is becoming lighter. Thus the shaft is becoming thinner. In this situation, in order to allow the shaft to have a desired rigidity and strength, the shaft is provided with a so-called thin multi-layer construction in which each prepreg layer is thin and the number of turns of prepregs is increased and a construction in which a reinforcing layer is partly provided.
However, the thin multi-layer construction is insufficient for preventing reduction in the strength of the lightweight shaft and causes the rigidity of the shaft to change. Therefore the thin multi-layer construction does not allow the shaft to have a target property value. Further since thin prepregs are used, a larger number of prepregs is wound on a mandrel. Thus a long prepreg-winding time is required, which leads to a workability and which causes air to penetrate into the space between adjacent prepregs easily. Consequently there is a possibility that the performance of the shaft such as its strength and the like is adversely affected thereby. Therefore various proposals are made to improve the strength and the like of the lightweight shaft made of the fiber reinforced resin.
In Japanese Patent Application Laid-Open No.59-20181, there is disclosed a golf club shaft made of fiber reinforced resin in which to eliminate variations of a player's feeling, the bending modulus of elasticity of the outermost layer and that of the inner layer in contact with the outermost layer are specified.
In Japanese Patent Application Laid-Open No.5-49718, a golf club made of carbon fiber reinforced resin is disclosed to obtain a golf club lightweight and highly rigid. The golf club has the inner layer, the outer layer, and the reinforcing layer. In the golf club, the weight a unit length of the shaft, the rigidity thereof, and the twist angle thereof are specified. More specifically, the inner-side prepreg is more elastic than the outer-side prepreg, and the construction of dividing the outer layer into two layers (inner portion and outer portion) is proposed.
However, in the golf club shaft disclosed in Japanese Patent Application Laid-Open No.59-20181, only the bending modulus of elasticity of the outermost layer and that of the inner layer in contact with the outermost layer are specified. Thus in the case where a multi-layer construction which is the normal layering construction is adopted, the strength of the golf club shaft is adversely affected in dependence on the tensile modulus of elasticity of an intermediate layer and the golf club shaft does not twist smoothly. Thus a player has a bad feeling.
In the golf club disclosed in Japanese Patent Application Laid-Open No.5-49718, the relationship between the magnitude of the tensile moduli of elasticity of the adjacent layers and the strength thereof is not specified. Thus in the case where the prepreg having a higher tensile modulus of elasticity than that of the outer layer is used in the inner portion of the outer layer, the strength of the golf club shaft may be insufficient.